The present invention involves the maintenance of stability of gussets in thermoplastic bags following the formation of such gussets during manufacture of the bag. Gusseted plastic bags are typically manufactured from a substantially continuous sheet of dual-walled plastic film integrally connected along at least one continuous edge. The edge of the film web opposite the integrally connected edge may be open-sided or openable along fastening means such as a zipper. Further, the film web may be tubular in form.
A gusset is formed when an integrally connected film web edge is folded inward to form folds along that edge as illustrated in Wamsley, U.S. Pat. No. 2,961,930. The film web may then be severed and sealed to form individual gusseted bags. Upon filling of the gusseted bag, the gusset folds expand to reveal side or bottom walls of dimension commensurate with the width of the folds. Gusseted plastic bags have advantages over ungusseted plastic bags in that they may be stood upright upon filling or have rectangular sides.
Numerous apparatus which form gussets in plastic film web are known in the prior art. Wech, U.S. Pat. No. 3,678,813 and Knuetter, U.S. Pat. No. 2,283,069 relate apparatus for gusseting an open-sided film web. Hummel et al., U.S. Pat. No. 4,526,565 and Rochla, U.S. Pat. No. 3,587,907 relate an apparatus for gusseting a tubular film web.
A common processing problem encountered is maintainance of stability of the gusset after formation thereof. The term stability refers to the extent to which the gusset fold configuration is maintained after the gusset is introduced into the film web. The depth and alignment of the gusset folds with respect to each other and the entire web must be substantially maintained until the severing and sealing of the film web into individual bags. Gusset stability after severing and sealing is not a problem since the gusset structure is permanently sealed into the resulting bag.
A common method of enhancing gusset stability is passing the film web through mechanical means to press the gusset folds into the film web and maintain the resulting creases thereafter by providing an adequate level of process film web tension in the machine direction. These measures may be inadequate since many types and thicknesses of plastic film web, particularly polyethylene films less than two-thousandths of an inch in thickness, have negligible creasibility.
Another common processing problem encountered with gusseted bags is the formation of leakage points along the side welds of individual bags after severing and sealing of the bag film web. The formation of leakage points is due to incomplete heat sealing of the edges of the walls of the bag including those of the gusset. Air or liquid may leak out of the bag leakage points if the bag is filled with either or both. A particular problem leakage area along the side weld is at the top or the innermost point of the inward fold of the gusset.
Accordingly, there exists a need for a process for gusseting a continuous, dual-walled thermoplastic film web wherein the stability of the gusset is maintained and the incidence of leakage points along the side welds of individual bags formed from the gusseted film web is minimized.